1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device in general, and more particularly, to an LCD device and a method for fabricating the same, to decrease step coverage in color filter layers of LCD devices having different models formed on one mother substrate.
2. Discussion of the Related Art
Demands for various display devices have increased with development of an information society. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), and vacuum fluorescent displays (VFD). Some species of flat display devices have already been applied to displays for various equipment.
Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to advantageous characteristics, such as, for example, thin profile, lightness in weight, and low power consumption, whereby the LCD devices provide a substitute for Cathode Ray Tubes (CRT). In addition to mobile type LCD devices such, for example, as a display for a notebook computer, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology having applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can provide a high quality picture, including characteristics such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
In general, an LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second substrates bonded to, and separated from each other at a predetermined interval, and a liquid crystal layer formed between the first and second glass substrates. To maintain the predetermined interval between the first and second substrates by spacers, the first and second substrates are bonded to each other by a sealant, and then the liquid crystal layer is formed between the first and second substrates. Meanwhile, alignment layers are respectively formed on opposite surfaces of the first and second substrates, and are rubbed to align the liquid crystal layer.
Hereinafter, a related art LCD device will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view of a related art LCD device. As shown in FIG. 1, the related art LCD device includes a first substrate 1a, a second substrate 1b, and a liquid crystal layer 3. The first and second substrates 1a and 1b are bonded to each other at a predetermined interval, and the liquid crystal layer 3 is formed between the first and second substrates 1a and 1b by injection of liquid crystal.
In more detail, a plurality of gate lines G are formed on the first substrate 1a at fixed intervals in one direction, and the plurality of data lines D are formed at fixed intervals perpendicular to the plurality of gate lines G, thereby defining a plurality of pixel regions P (also referred to herein as unit pixel regions). In each of the pixel regions P, there is a pixel electrode 6 being overlapped with a predetermined portion of the gate line G of the adjacent pixel region P. Also, a plurality of thin film transistors T are formed at respective crossing portions of the plurality of gate and data lines, wherein each thin film transistor T is turned on/off by a scan signal of the gate line G so as to apply a data signal of the data line D to the corresponding pixel electrode 6. The second substrate 1b includes a black matrix layer 7 that excludes light from regions except the pixel regions P of the first substrate 1a, R/G/B color filter layers 8 corresponding to the pixel regions P to display various colors, and a common electrode 9 to obtain a picture image. In this state, alignment layers (not shown) are formed on opposite surfaces of the first and second substrates 1a and 1b, and are rubbed to align the liquid crystal layer 3.
In the related art, the thin film transistor T is comprised of a gate electrode projected from the gate line G, a gate insulating layer (not shown) on an entire surface of the first substrate 1a, an active layer formed on the gate insulating layer above the gate electrode, a source electrode projected from the data line D, and a drain electrode being opposite to the source electrode. Also, in the related art, the pixel electrode 6 is formed of a transparent conductive metal material having great transmittance, for example, indium-tin-oxide (ITO).
In the related art LCD device, the liquid crystal layer 3 is driven by a vertical electric field between the pixel electrode 6 of the first substrate 1a and the common electrode 9 of the second substrate 1b. This has the advantageous characteristics of great transmittance and high aperture ratio. However, it has the problem of a narrow viewing angle. Accordingly, in order to overcome the problem of the narrow viewing angle of the related art LCD device, an IPS mode LCD device using a parallel electric field to two substrates has been developed.
FIG. 2A is a plane view of a unit pixel region for a thin film transistor array substrate of a related art IPS mode LCD device. FIG. 2B is a plane view of a unit pixel region for a color filter array substrate of a related art IPS mode LCD device. FIG. 3 is a cross sectional view along I-I′ of FIG. 2A and FIG. 2B.
As shown in FIG. 2 and/or FIG. 3, the related art IPS mode LCD device includes a first substrate 10a, a second substrate 10b, and a liquid crystal layer 80. On the first substrate 10a, a common electrode 50 and a pixel electrode 40 are formed in parallel. Also, a color filter layer 11 is formed on one surface of the second substrate 10b being opposite to the first substrate 10a. Then, the liquid crystal layer 80 is formed between the first and second substrates 10a and 10b. 
In more detail, as shown in FIG. 2A and FIG. 3, the first substrate 10a includes a plurality of gate lines G, a plurality of data lines D, a plurality of thin film transistors T, a common line CL, a plurality of common electrodes 50, a plurality of pixel electrodes 40, and a storage electrode 15. At this time, the plurality of gate lines G are formed on the first substrate 10a, and the plurality of data lines D are formed perpendicular to the gate lines G, thereby defining a plurality of pixel regions. Also, the plurality of thin film transistors T are formed at respective crossing portions of the gate and data lines. Then, the common line CL is formed in parallel with the gate line G within the pixel region P. The plurality of common electrodes 50 are diverged from the common line CL, and are formed in parallel with the data line D. Furthermore, the pixel electrode 40 is connected with a drain electrode DE of the thin film transistor T, and is formed between each of the common electrodes 50 in parallel. The storage electrode 15 extending from the pixel electrode 40 is formed on the adjacent gate line G. Herein, the thin film transistor T further includes a gate electrode GE and a source electrode SE.
As shown in FIG. 2B and/or FIG. 3, the second substrate 10b includes a black matrix layer BM, color filter layers 11, and an overcoat layer OC. The black matrix layer BM is formed on an entire surface of the second substrate 10b except the pixel regions, and the color filter layers 11 are formed in correspondence with the pixel regions. Also an overcoat layer OC is formed on the entire surface of the second substrate 10b including the color filter layers 11, so as to decrease step coverage between the color filter layers of the adjacent pixel regions, and to prevent the color filter layers 11 from being contaminated by pigment.
In the aforementioned IPS mode LCD device, the liquid crystal layer 80 is driven by an electric field parallel to the two substrates, generated between the common electrode 50 and the pixel electrode 40 formed in parallel with the pixel region of the first substrate 10a. 
A method for fabricating the color filter layer 11 of the second substrate 10b of the related art IPS mode LCD device will be described as follows.
FIG. 4A to FIG. 4C are cross sectional views of structures achieved by a fabrication process for the color filter layer of the related art IPS mode LCD device, which will be explained with reference to three pixel regions corresponding three color filter layers of R, G, and B.
First, as shown in FIG. 4A, the second substrate 10b, defined by repetitively forming the first, second, and third pixel regions, is prepared. Then, chrome or resin is deposited on the second substrate 10b, and is patterned by photolithography, thereby forming the black matrix layer BM on the entire surface of the second substrate 10b except the pixel regions.
Subsequently, as shown in FIG. 4B, red resist is coated on the entire surface of the second substrate 10b having the black matrix layer BM, and is patterned by photolithography, thereby forming a red color filter layer 11a in the first pixel region of the second substrate 10b. 
Then, as shown in FIG. 4C, green resist is coated on the entire surface of the second substrate 10b having the red color filter layer 11a, and is patterned by photolithography, thereby forming a green color filter layer 11b in the second pixel region of the second substrate 10b.
After that, blue resist is coated on the entire surface of the second substrate 10b including the red and green color filter layers 11a and 11b, and is patterned by photolithography, thereby forming a blue color filter layer 11c in the third pixel region of the second substrate 10b. 
Subsequently, the overcoat layer OC is formed on the entire surface of the second substrate 10b including the respective color filter layers 11a, 11b, and 11c, so as to decrease the step coverage of the respective color filter layers 11a, 11b and 11c. Traditionally, if a layer of material is to be deposited on top of a thin strip of material, step coverage is defined as the ratio of film thickness along the walls of the step to the film thickness at the bottom of the step. Applying this definition to FIG. 4, step coverage of the respective filter layers 11a, 11b and 11c is the structural thickness (in a vertical direction) difference of the respective color filter layers 11a, 11b, and 11c and the black matrix layer BM located between those respective color filter layers at the foot of the step.
Meanwhile, as each of the color filter layers 11a, 11b, and 11c becomes thicker, and the step coverage of the color filter layers 11a, 11b, and 11c (i.e., structural thickness difference (in the vertical direction) between the color filter layers 11a, 11b, and 11c and the black matrix layer BM located between the respective color filter layers) becomes larger, it is required to thicken the overcoat layer OC. Thus, the transmittance of the IPS mode LCD device is lowered. In the process for forming the respective color filter layers 11a, 11b, and 11c, it is important to decrease the thickness in each of the color filter layers 11a, 11b, and 11c, and to minimize the step coverage of the respective color filter layers 11a, 11b, and 11c. 
To improve productivity of the fabrication process for the LCD device, instead of forming one LCD panel on one mother glass substrate, a plurality of LCD panels are formed on one mother glass substrate at the same time. Also, a plurality of LCD panels of the same model may be formed on one mother glass substrate. Or, a plurality of LCD panels of the different models may be formed on one mother glass substrate. Before injecting liquid crystal, the mother glass substrate may be cut into the unit LCD panels. Then, the fabrication process is performed with respect to each of the LCD panels.
In case the LCD panels have different models, unit pixel regions of the respective LCD panels may have different sizes. Also, even though the LCD panels have the same size, the unit pixel regions of the LCD panels may have different sizes according to resolution.
FIG. 5 is a plane view of a multi-model LCD device for designing a plurality of LCD panels of the different models on one mother substrate.
For example, as shown in FIG. 5, on the assumption that first, second, and third LCD panels 60a, 60b, and 60c of different models are formed on one mother substrate 60, a black matrix layer and a color filter layer are formed on each portion corresponding to the respective LCD panels 60a, 60b, and 60c. 
If, for example, the first LCD panel 60a has a low resolution, and the second LCD panel 60b has a high resolution, then the unit pixel region of the first LCD panel 60a having the low resolution is larger than the unit pixel region of the second LCD panel 60b having the high resolution. Thus, the thickness of pigment for the color filter layer 70a formed in the unit pixel region of the first LCD panel 60a having the low resolution is different from the thickness of pigment for the color filter layer 70b of the unit pixel region of the second LCD panel 60b having the high resolution, thereby generating step coverage between the color filter layers 70a and 70b of the different models.
That is, as shown in (a) of FIG. 5, the color filter layer 70a formed in the unit pixel region of the first LCD panel 60a having a low resolution is thinner than a color filter layer 70b formed in the unit pixel region of the second LCD panel 60b having the high resolution. Accordingly, the completed LCD devices may have the different chromatic coordinates.
As described above, the IPS mode LCD device has a thick overcoat layer OC so as to decrease the step coverage between the color filter layers 70a and 70b. However, as the overcoat layer OC becomes thicker, the transmittance is lowered.